1. Field of the Invention
The present invention relates to a high silicon steel and a method thereof.
2. Description of the Related Arts
Soft magnetic properties of silicon steel sheets which are used as a core material of electromagnetic induction equipment are improved with the increase of the added amount of Si. It is known to give maximum magnetic permeability of the silicon steel sheet at around 6.5 wt. % of Si content. If, however, the Si content increases to 4 wt. % or more, the workability of the steel sheet rapidly deteriorates. Therefore, it was accepted that the ordinary rolling method cannot produce high silicon steel sheet on a commercial scale.
As a method for commercially manufacturing high silicon steel sheet containing 4 wt. % or more Si by solving the above-described problem on workability, the siliconizing method is disclosed in Japanese unexamined patent publication No. 62-227078. The siliconizing method comprises the steps of: reacting a thin steel sheet containing less than 4 wt. % Si with SiCl.sub.4 at an elevated temperature to penetrate Si into the steel sheet; and diffusing the penetrated Si in the sheet thickness direction, thereby to produce a high silicon steel sheet. For example, Japanese unexamined patent publication No.62-227078 and Japanese unexamined patent publication No. 62-227079 subject a steel sheet to continuous siliconizing treatment in a non-oxidizing gas atmosphere containing 5 to 35 wt. % SiCl.sub.4 at a temperature of from 1023 to 1200.degree. C., thus obtaining a coiled high silicon steel sheet.
Generally, the siliconizing treatment uses SiCl.sub.4 as the raw material gas to supply Si. The SiCl.sub.4 reacts with the steel sheet in accordance with the reaction equation given below. Si penetrates into the surface layer of the silicon steel sheet. EQU SiCl.sub.4 +5Fe.fwdarw.Fe.sub.3 Si+2FeCl.sub.2
The Si thus penetrated into the surface layer of the steel sheet diffuses in the sheet thickness direction by soaking the steel sheet in a nonoxidizing gas atmosphere containing no SiCl.sub.4.
A continuous siliconizing line for continuously siliconizing a steel sheet by the process described above has heating zone, siliconizing zone, diffusing and soaking zone, and cooling zone, from inlet to exit thereof. That is, the steel sheet is continuously heated in the heating zone up to the treatment temperature, and the steel sheet is reacted with SiCl.sub.4 in the siliconizing zone to let Si penetrate into the steel, then the steel sheet is continuously heat-treated in the diffusing and soaking zone to diffuse Si in the sheet thickness direction, and the steel sheet is cooled in the cooling zone to obtain a coiled high silicon steel sheet.
Conventional continuous annealing line maintains the oxygen concentration and dew point in the annealing furnace at a constant level to suppress the oxidization on the surface of steel sheet. As to the intrafurnace atmosphere of a continuous siliconizing line, Japanese unexamined patent publication No.6-212397 points out a problem that the oxidization occurs at surface and at grain boundary of the steel sheet and bending workability of product is deteriorated when the steel is subjected to siliconizing and diffusion treatment in an atmosphere having a water vapor concentration corresponding to dew point of -30.degree. C. or more. Therefore, the patent publication proposes a method for continuously manufacturing high silicon steel sheet having excellent bending and punching workability wherein the oxidization at surface and grain boundary of the steel sheet is restrained and products having favorable workability are manufactured. According to the method, the intrafurnace atmosphere is controlled so as to satisfy the following conditions:
oxygen concentration; 45 ppm or less,
dew point; -30.degree. C. or less,
O.sub.2 !, H.sub.2 O!; H.sub.2 O !.sup.1/4 .times.O.sub.2 !&lt;80,
wherein O.sub.2 ! is oxygen concentration expressed by ppm and H.sub.2 O! is water vapor concentration expressed by ppm.
A method for controlling the intrafurnace atmosphere to establish the above-described conditions is the method using the strong reducing power of carbon. The continuous siliconizing line is held at 1023.degree. C. or more to carry out the penetration and diffusion of Si. When carbon exists in the steel sheet within the temperature range, the oxygen and water vapor in the furnace react with the carbon to form CO, thus enabling the control of intrafurnace atmosphere that was proposed by unexamined Japanese patent publication No. 6-212397.
When, however, that type of method was applied to control the intrafurnace atmosphere to manufacture high silicon steel sheets, the workability of products was found to be deteriorated even when the oxidization at surface and grain boundary of the steel was suppressed.
On the other hand, as described before, it was accepted that a high silicon steel sheet containing 4 wt. % or more Si cannot be produced by rolling method. However, Japanese unexamined patent publication No. 63-35744, for example, proposed to roll a steel sheet under the control of rolling temperature and rolling reduction. That type of technology enables to conduct rolling.
To use a high silicon steel sheet practically as a core material for electromagnetic induction equipment, however, a secondary working such as punching, bending, shearing is required to apply to the steel sheet. Thus, there is a problem that, even if a high silicon steel sheet is manufactured by the rolling method through the control of rolling temperature and rolling reduction, the steel sheet cannot be worked to form a core for electromagnetic induction equipment owing to the poor secondary workability.